The increase of price of natural gas has pushed the development of gas turbines in the direction of considering alternative fuels such as the so called synthesis gases. These gases typically come from gasification processes of solid feedstock such a coal, pet coke or biomass. The use of synthesis gas in gas turbines involves a much larger volumetric flow injection of fuel than standard natural gas. In order to achieve the very low NOx values that are typical of modern gas turbines it is needed for the combustors to run in premixed mode. The fact of having such a large volumetric flow for the synthesis gas poses serious problems in firing them in a premixed mode. Another problem arising is the different reactivity of these fuels which can be different from natural gas; especially when there is a significant fraction of hydrogen the reactivity tends to be high, and this constitutes a problem as it aggravates the danger of flashback. These translate into the fact that a much larger fuel flow through the air passages leads to an increase in pressure drop, and that mixing between the air and fuel tends to be poor. The latest is mainly due to the fact that large volumetric flow rates also require large nozzles, which typically leads to bad mixing, and hence to high NOx emissions.
PCT/US2009/001336 shows an additional stage in the fuel injector and swirler assembly for injecting low calorific (LC) fuels. This stage supplies two additional rows of fuel injectors. The fuel injectors are implemented in the vanes of the swirlers and inject the fuel in a jet-in-cross flow mode. To provide the space for the two additional rows of injectors, the vanes are elongated in their upstream direction. Moreover the hub diameter has been increased to enlarge the space for the supply fuel flows. The combustor has demonstrated its functionality in terms of flashback resistance and low dynamics. However, one problem with this design is that the capacity of the LC fuel passage is still relatively small, and that in order to keep the fuel side pressure drop as low as possible, the natural gas stages are also used for injecting the LC fuel. This makes the system relatively complicated in terms of fuel supply splits. Even when splitting the LC-fuel over all stages, pressure drops are relatively large. Moreover, the injectors for the LC fuel are relatively small. Since these fuels are typically contaminated, a clogging issue could arise.